Optical fibers are used in a variety of applications such as telecommunications, illumination, fiber lasers, laser machining and welding, sensors, medical diagnostics and surgery.
A typical standard optical fiber is made of transparent material. It is uniform along its length, and has a cross-section of varying refractive index. For example, the transparent material in the central region, i.e. the core, may have a higher refractive index than the transparent material in the outer region, i.e. the cladding. Light is confined in or near the core and guided along the length of the optical fiber by the principle of total internal reflection at the interface between the core and cladding.
A microstructured optical fiber (MOF), also known as a holey fiber, a photonic crystal fiber or a photonic bandgap fiber, differs from a standard waveguide fiber in that it has a cross-section microstructured from two or more materials. A microstructured fiber has a cladding running the length of the fiber that is microstructured from two or more materials most commonly arranged periodically over much of the cross-section. For example, a microstructured optical fiber may have a cladding made of a transparent material in which a periodic array of holes extends longitudinally, the holes being arranged periodically over much of the cladding cross-section and filled with material which has a lower refractive index than the transparent material of the rest of the cladding, and a core of transparent material consisting of a break in the periodic array of the cladding. MOFs can guide light according to one of two mechanisms. In a microstructured optical fiber with a solid core, or a core with a higher average refractive index than the microstructured cladding, light may be guided along the core by the same index-guiding mechanism of total internal reflection as in standard optical fibers or by a mechanism based on photonic-band-gap effects. With total-internal-reflection guidance, MOFs can have a much higher effective-index contrast between core and cladding, and therefore can have much stronger confinement for specialized applications. With photonic-band-gap guidance, light is confined by a photonic bandgap created by the microstructured cladding. A properly designed bandgap can confine light in a hollow core or a core of lower refractive index than the cladding.
In general, an optical fiber may be multi-mode or single-mode. A multi-mode fiber allows for more than one mode of the light wave, each mode travelling at a different phase velocity, to be confined to the core and guided along the fiber. A single-mode fiber supports only one transverse spatial mode at a frequency of interest. Given a sufficiently small core or a sufficiently small numerical aperture, it is possible to confine a single mode, the fundamental mode, to the core. Single-mode fibers are preferred for many applications because the problem of intermodal dispersion encountered by multi-mode fibers is avoided, and the intensity distribution of the light wave emerging from the fiber is unchanged regardless of launch conditions and any disturbances of the fiber.
For some applications, it is advantageous to carry as much optical power as possible. However, if the light intensity within the fiber exceeds a certain threshold, the material from which the fiber is made will suffer irreversible damage. Increasing the diameter size of the core of the fiber reduces the intensity of the light for a given power and allows a greater power to be carried. Using a larger core fiber also helps to reduce the non-linear effects that appear at high power.
For example, in the field of high-power lasers and amplifiers, the onset of adverse non-linear effects can severely degrade the spectral content and limit the power output of the laser source. Using a single-mode large-mode-area active fiber as the amplifying medium is a relatively easy solution to the problem of non-linear effects which can be detrimental to the operation of the laser.
Most microstructured optical fibers that are reported in the prior art literature—for example, in U.S. Pat. No. 6,334,019 (Birks et al.), U.S. Pat. No. 6,603,912 (Birks et al.), and U.S. Pat. No. 6,888,992 (Russell et al.)—consist of a cladding which has embedded along its length a substantially periodic array of holes and a core defined by the absence of at least one hole in the array, essentially, a triangular lattice of holes surrounding a central defect constituted by the absence of at least one of the holes in the lattice structure. One particular implementation of this design is the case where a seven-missing-holes defect defining a core is surrounded by a triangular lattice of holes defining a microstructured cladding. This is for example reported in Limpert et al., “Low non-linearity single-transverse-mode ytterbium-doped photonic crystal fiber amplifier”, Opt. Express 12(7), 1313 (2004) and in J. Limpert et al., “High-power rod-type photonic crystal fiber laser”, Opt. Express 13(4), 1055 (2005)]. By using such designs, a single-transverse-mode large-mode-area fiber that allows the transportation of high optical power while minimizing the non-linearity can be devised. Core diameters of 45 μm or greater may be achieved with such designs. Such large-mode-area fiber designs are particularly important in the field of high power lasers, where the core of the fiber is doped with active ions permitting a laser effect. However one drawback of the proposed triangular lattice structure of the cladding for the microstructured optical fibers is the quality of the mode profile of the light beam, more particularly the non-circular form of the guided mode profile, which stems from a central core having a cross-section which is more hexagonal than circular. Such sub-optimal beam quality can be detrimental for applications, such as precision laser surgery or micromachining, where light beam quality, i.e. a circular mode profile, is critical.
There is therefore a need for a microstructured optical fiber that enhances the circularity of the profile of the guided light mode.